What the Marines will be doing in Darwin has not yet been adequately described, though fingers are, of course, being pointed towards China, which has become more assertive in the seas near their coast. China has had what we would diplomatically call “incidents” with both Japanese and Vietnamese ships in the South China Sea in the past two years; it's likely not a coincidence that these incidents have occurred as each country explores the seabed for oil and natural gas deposits. And then there is the launch earlier this year of China's first aircraft carrier. Frankly, I have a hard time getting that worked up about a second-hand, Soviet-era boat from Ukraine, but others point to the ship, and China's renaming it the Shi Lang after the Chinese admiral who conquered Taiwan, as subtle signs of their aggressive intentions in the region.
On one hand, it is hard to see China actually going to war with any of its neighbors. China seems to have learned the lesson from the Soviet Union that trying to build and maintain an empire through military force is a sure route to bankruptcy. Instead, China has followed the post-Soviet model laid down by Russia of trying to dominate countries through economics, either as suppliers of raw materials or consumers of your goods. In that respect, war would be bad for business, and it's worth noting that China and Australia do a lot of business together. But on the other hand, there's demographics. Thanks to China's “one child” policy and cultural preference for boys, the male-female ratio is seriously out of whack, one statistic I have seen puts it at 88 women for every 100 men (typically the ratios are near 50/50 with a slight lean towards women). Historically, societies with male/female ratios of this scale have been far more likely to go to war, since war gives unattached males something to do.
And there's always the modern American policy fallback position of anti-terror operations. The Obama Administration has quietly, though aggressively, stepped up anti-terror operations by drone aircraft and US special forces around the globe, for example places like Ethiopia and the Seychelles are now bases for US drones, while military advisers were recently sent to Uganda. Marines in Darwin would be ideally based to carry out operations in Indonesia, the world's largest (by population) Muslim nation and one that is not unfamiliar with Islamic extremism.
If nothing else, the basing of US Marines in Darwin will help to strengthen ties between the US and Australia, a nation that in recent years has been increasingly seeing itself more and more as part of Asia.